Yeah, if the rused area is flat, a simple sheetmetal insert cut to fit will work. If the rust is larger or around the bend of the cab, then it would be better to buy a lower cab corner repair panel. Cut away the original corner area to fit the new panel. Either one will require welding and smoothing out then re-painting. That's the best way to fix rust-cut it completely out.
There's many sources for the patch panels, just check the ad section of any truck magazine for sites. CCG Truck&Van, Raybuck, etc.